Basic Information

It can refer to a specific old con: selling a bar of lead (that's been gold plated or just painted gold) at the price a bar of solid gold would fetch.

Or, it can function as a more general term referring to other scams involve selling a tangible item for more than it is worth. In this sense it might describe a scam that is more specifically an example of one of these other categories of con:

When not related to confidence games, goldbricking can also refer to the practice of taking excessive care for one's own comfort, safety and resources and avoiding any effort, risk or exertion as far as possible. In effect, this latter type of "goldbricker" treats himself as an object of undue worth…

Sources

Game and Story Use

In a game where Alchemy is at least theoretically possible, it might be hard to tell whether that bar of lead has been turned to gold, or just coated in it. If it takes a high-level spell to actually transmute a bar, and a lesser magic to simply coat one or otherwise fake it, there's some possibility for a mix of both effects within the scheme. For example, if using a spells per day system, the alchemist might be only able to transform one bar per day (or per expensive material component), but is able to use illusions (or other magic) to make several dozen bars a day look golden long enough to close the deal.

"Fairy gold" operates on a similar principle, with worthless items like pebbles and leaves enchanted so that they appear to be valuable. After a certain amount of time passes, or when the items come in contact with a disenchanting agent like cold iron, the glamour fades and the greedy mortal is left with a sack full of acorns instead of the gold pieces he thought he had.

An artifact made of Tumbaga might be passed off as solid gold in such a scam.

The "goldbrick as character" type is likely to be an annoying PC (although some players inadvertantly demonstrate such tendencies in their playing style) but would make an amusing patron … plenty of work, but poorly paid.